What could have been the hardest question?

The hardest question that has not been answered yet is that why Donut have holes? It was a questioned asked to me during a job interview discussion. I had conducted web searches about it and nobody really knows the exact answers but all are speculations.

well.! for me the hardest question would be how actually does the human brain works!?

the hardest question is why..there must a reason after it..

who is the first person to create the tyre? i mean the concept of tyre, rock tyre, wooden tyre...

A lot my friend. The scientist in particular keeps on experimenting and studying not only to discover new things also to answer life's difficult question. Question like "are we alone in the universe?" , "what's is the origin of the universe?" . There is really a lot of question that is left unanswered.